Methods of controlling the drive motor of elevator installations can differ, for example, in the form of speed control and in the form of detection of the position of the elevator cage.
In the case of elevator installations for high demands with respect to travel speed and transport capacity the position of the elevator cage can be detected by an absolute position measuring system, which supplies to the elevator control information, from which the elevator control recognizes the current position of the elevator cage. The travel speed is regulated in correspondence with a trip/speed profile, the course of which is determined in dependence on the trip distance between a start position and a destination position before the start of the trip.
In the case of elevator installations for moderate demands with respect to travel speed and transport capacity the position of the elevator cage is often detected by a position detection system with a travel transmitter. Such a travel transmitter is often constructed as an incremental rotary encoder and is driven by the movement of the elevator cage by means of a transmission mechanism. In one often-employed form of embodiment, an incremental rotary encoder is coupled with the rotating axle of the cable pulley of a speed limiter, wherein a wire cable transmits the movement of the elevator cage to the cable pulley of the speed limiter and thus forms the mentioned transmission mechanism.
A travel transmitter supplies to the elevator control signals from which the elevator control can derive trip distances, speed and acceleration of a movement of the elevator cage. The information about the position of the elevator cage can be detected by summation of the detected trip distances. It can therefore be falsified or lost, for example as a consequence of disturbances in the signal transmission or interruptions in the power supply, which can require measures for reinstatement of the correct position in the position detecting system.
Such a position detecting system for an elevator cage of an elevator installation is known from WO 01/70613. In the described equipment the elevator control registers the current position of the elevator cage over the entire trip distance on the basis of signals of an incremental rotary encoder coupled with the cable pulley of a speed limiter and thus with the movement of the elevator cage. However, noise pulses and, in particular, slips in the cable drive coupling the movement of the elevator cage with the incremental rotary encoder produce deviations between the instantaneously registered position of the elevator cage ascertained on the basis of the signals of the incremental rotary encoder and the actual instantaneous cage position. In order to provide compensation for the effect of such disturbing influences, the instantaneously registered position of the elevator cage is corrected on arrival of the elevator cage at a destination stopping point and/or movement past intermediate stopping points. This is carried out in that, with the help of a stopping point sensor mounted on the elevator cage, a respective stopping point marking associated with a specific stopping point is detected, whereupon the position of the elevator cage instantaneously registered in the elevator control is corrected in correspondence with the stored stopping point position value associated with the respective stopping point. Moreover, the elevator control is conceived so that a stored stopping point position value is corrected when this gives repeated cause for significant corrections, which are effective in the same direction, of the instantaneously registered position of the elevator cage.
In the cited prior art, in which the slip corrections are undertaken only on reaching the stopping point marking of the destination stopping point, the movement of the elevator cage into the region of the stopping point marking has to be carried out at reduced travel speed. This is due to the fact that the slip arising in the coupling between the movement of the elevator cage and the movement of the incremental rotary encoder can lead to such a large deviation of the instantaneously registered elevator cage position from the actual instantaneous elevator cage position that the position-dependent travel speed, which is present on movement of the elevator cage into the region of the stopping point marking of the destination stopping point, of the elevator cage can be so high that braking until reaching the destination stopping point position is no longer possible. Such a situation leads to disruptions of the normal operation and can even lead to shutdown of the elevator installation. The stated slip-dependent deviation can, however, also be of such a kind that the travel speed of the elevator cage present when the elevator cage moves into the region of the stopping point marking of the destination stopping point is already too low, so that in order to reach the destination stopping point position an extended trip at low speed and correspondingly increased journey time is required.